The current manufacturing technology for forming solder bumps on the pads of organic substrates is the solder paste stencil printing method. The stencil printing method of solder paste is suitable for forming solder bumps on the pads of organic substrates at low cost because additional lithography or vacuum processing is not needed. However, due to the significant solder volume reduction after reflow, the stencil printing method cannot be easily extended to fine pitch applications under 150 microns.
Other solder application technologies, such as the micro-ball mounting method, eliminate the volume reduction problem by using preformed solder balls. However, the cost of preformed solder balls significantly increases with size reduction for finer pitch applications, because the uniformity of preformed solder balls decreases as the size of the ball decreases. Difficulties also arise in attempting to form solder bumps on C4 (Controlled Collapse Chip Connection) pads and capacitor pads in a one step process. The capacitor pads need more than 20 times the solder volume of the C4 pads, so an additional stencil printing method is required for the capacitor pads after the ball mounting method forms the solder bumps on the C4 pads.
Injection molded solder (IMS) with double layer decals can be used for forming solder bumps on organic substrates because a protruding solder structure can be created by peeling of one layer decal after the IMS process. U.S. Pat. No. 5,673,846 of Gruber discloses a solder anchor decal and method. A solder decal is produced by a method wherein a decal strip having a plurality of anchor holes is aligned with a mold having a plurality of cells. Liquid solder is injected into the anchor holes and mold cells, and is then allowed to cool to solidify therein. The mold may be separated from the decal strip to form the solder decal containing solder beads each having a stem mechanically joined to the strip at respective ones of the anchor holes. Various forms of the solder decal are used for transferring the solder beads to a substrate or chip, or effecting temporary connections for conducting burn-in and testing, or accommodating thermal mismatch for example.
U.S. Pat. No. 5,244,143 of Ference et al. discloses an apparatus and method for injection molding solder and applications thereof. An apparatus and method are described for injection molding solder mounds onto electronic devices. The apparatus has a reservoir for molten solder which is disposed over a cavity in an injection plate. The injection plate is disposed over a mold having an array of cavities therein into which solder in injection molded. The mold is disposed over a workpiece, such as a semiconductor chip or a semiconductor chip packaging substrate. The cavities in the mold are aligned with electrical contact locations on the chip or substrate. The workpiece is heated and the molten solder is forced under gas pressure into the cavity in the injection plate disposed above the array of cavities in the mold. The molten solder is forced into the array of cavities in the mold. The injection plate is advanced to slide over the mold to wipe away the excess solder above the mold at a plurality of wiping apertures in the injection plate. The injection plate is further advanced to a location having a nonsolder wettable surface at which location the injection plate is removed. The mold is then removed to leave solder mounds disposed on the workpiece. The workpiece can be a semiconductor chip, a semiconductor chip packaging substrate or a dummy substrate onto which the injected molded solder adheres such as a polymer layer to form a carrier substrate for a solder mound array which can be subsequently transferred to a substrate such as a semiconductor chip or a semiconductor chip packaging substrate. The apparatus and methods of the invention can be integrated into an automated manufacturing system for depositing an array of solder mounds onto a substrate.